The antifolates, methotrexate, aminopterin, 10-deazaaminopterin and sulfasalazine are clinically useful in the treatment of rheumatoid arthritis. Toxicity, rather than efficacy, appears to be the major factor limiting the usefulness of the classical antifolates (i.e., methotrexate and 10-deazaaminopterin). The fact that folate supplementation of methotrexate-treated rheumatoid arthritis patients reduces toxicity without altering efficacy also suggests that inhibition of the drug's target enzyme, dihydrofolate reductase, is not complete and not essential for efficacy. Since polyglutamates of methotrexate are direct inhibitors of thymidylate synthase and folate dependent enzymes of purine biosynthesis, the efficacy of this agent may involve blockade of these pathways. We hypothesize that blockage of aminoimidazole carboxamide ribotide transformylase, the folate dependent enzyme responsible for the insertion of carbon 2 into the purine ring, produces an immunosuppression mediated by secondary inhibition of adenosine deaminase, and S-adenosyl homocystein hydrolase by aminoimidazolecarboxamide metabolites. This mechanism of immunosuppression may explain the clinical effect of methotrexate, 10-deazaaminopterin, an possibly sulfasalazine. Since purine biosynthesis is a fundamental process, blockading this pathway may also decrease leukotriene production and interleukin-1 expression, which also could contribute to the efficacy of methotrexate.